Hi Kernos,
this is an interesting question and one to which there are several possible responses. The most bald is: 'no', but that needs to be qualified.

First, Welsh orthography developed over the centuries, first in manuscript and then in print. The only times I have seen an eth used for dd are in 19th and early 20th century grammars of Welsh, usually designed for linguists; even there the letter used is an ordinary d with a line through it, not the more curly delta-with-a-line that you see used for Old English, Middle English, Norse etc. But this never caught on.

I have checked in the relevant literature - as far as I can see, the eth has never been used for the sound -dd- at any stage (with the minor exception noted above.)

Basically, in Old and Middle Welsh, if you want to indicate a dd sound you write -d(-).

If you want to indicate a d- sound medially or finally, on the other hand, you write t.

byt --> ModW byd 'world'

This is essentially the Middle Welsh system, though certain scribal schools tried various peculiar innovations. For example, the scribe of Peniarth 20 (c. 1330) used a ligatured qu- to represent -dd. Barmy. That never caught on. (Incidentally, the Welsh never seem to have used the cornish dh for dd.)

Thank God by the late medieval period, dd comes in and everything is much clearer.

Ah! In this passage you quote, the eth is in fact a typo for the letter usually represented in modern editions as a '6' - which can stand for either v or w or u in Middle Welsh. So for example the first word is 'ysg6yt'= 'ysgwyd', 'shield'. The text is going on about these noble guys from the Old North having three hundred swords and shields and spears whatever business they were on. (The last one is wae6 = wayw, lenited form of gwayw, 'spear'.)

One of the problems with medieval script in welsh was that they had three sounds, /v/, /w/ and /u/ [the latter of which was tending more and more to be pronounced like /i/ in the Medieval period] but originally only two letters to represent those sounds, v and u. But in the 13th C Welsh scribes tended to write v in two different ways; at the beginning of a word or sentence it curled outward, but internally it curled inward like a 6. Then by the 14th C, they had developed the 6 into a separate letter which could be used at any point in a word.

To give you an example of MW orthography, here is the first line of Pwyll in Mod W orthography, then in the MW one:

So not -dd at all, it's just a bad formatting on that particular website.

BTW, Annwyl is a bit out of date - for good stuff on Old Welsh the great scholar was Ifor Williams. Have a look at is 'The Beginnings of Welsh Poetry'; he also produced what remain the standard editions of the 'core' Medieval Welsh texts: Canu Taliesin (availbale in an English version trans. by J Caerwyn Williams), Canu Aneirin, Canu Llywarch Hen, and the Pedeir Keinc Y Mabinogi. Only the 'Poems of Taliesin' is much use if you don't read W though.

Kernos wrote:Do or did Welsh ever use the character 'ð' or eth for 'dd'?

What I found looks like “6” in italics and the character represents a letter not a number.

From the Introduction of: Trioedd Ynys Prydein, The Triads of the Island of Britain, Third Edition, edited by Rachel Bromwich, University of Wales Press 2006, Manuscripts and Versions, pages XVI-XVII.

"... Orthography of Peniarth 16 [formerly Hengwrt 54] The normal spelling employed is that which is common to the Llyfr Gwyn, the Llyfr Coch, and other manuscripts on the thirteenth and fourteenth centuries. The following outline will indicate the main differences from Modern Welsh...

I read recently that <ð> was used a lot in Old English, and it was even used in Middle English. I suppose it's possible that writers with familiarity with the Old/Middle English alphabet could have used it to write in Welsh sometimes.

Written standards for Welsh were established in 1928:

Wikipedia wrote:In 1928 a committee chaired by Sir John Morris-Jones standardised the orthography of modern Welsh.

In 1987, a committee chaired by Professor Stephen J. Williams made further small changes. The conventions established by these committees are not adhered to by all modern writers.

Before that written standards could vary (as has been explained), which might have included <ð>, though perhaps due to the whims of an individual than traditional writing.

I think the most relevant bit that Wiki offers is the bit just before treegod's quotes.

Wikipedia wrote:The earliest samples of written Welsh date from the 6th century and are in the Latin alphabet (see Old Welsh). The orthography differs from that of modern Welsh particularly in the use of p, t and c to represent the voiced plosives /b, d, ɡ/ in the middle and at the end of words. Similarly, the voiced fricatives /v, ð/ were written with b and d.[2]
By the Middle Welsh period, this had given way to much variability: although b, d and g were now used to represent /b, d, ɡ/, these sounds were also often written as in Old Welsh, while /v/ could be denoted by u, v, f or w. In earlier manuscripts, moreover, fricatives were often not distinguished from plosives (e.g. t for /θ/, the sound now written with th).[3] The grapheme k was also used more commonly than in the modern alphabet, particularly before front vowels.[2] The disuse of this letter is at least partly due to the publication of William Morgan's Welsh Bible, whose English printers, with type letter frequencies set for English and Latin, did not have enough k letters in their type cases to spell every /k/ sound as k, so the order went "C for K, because the printers have not so many as the Welsh requireth";[4] this was not liked at the time, but has become standard usage.

From my studies I think that Welsh would originally have been more likely to use Latin letters (hence the B and D mentioned above) rather than Anglo-Saxon letters.

Last year I was studying medieval Welsh poetry and even though the facsimilies are not the best to work from (but all I had along with books) I'd honestly never seen that letter used.

Admitedly most of the extrant versions of from scriptoria in which the latin script was used along with the latin language. I'd never have imagined that it would have crept into written welsh. But obviously it has.

It certainly doesn't reflect modern conventions. English spelling conventions have varied greatly, and Welsh can too; without some "standard" people would use what they could.

This is just one example of eth used in Welsh, and I'm still not sure if the translation of the bible comes from pre- or post-1928. What would be interesting is to know when that translation was written, by who and also how prevalent eth is in written Welsh. Wikipedia is a bit limited in this respect.

John Morris Jones says "Welsh, in all its periods, has been written in the Latin alphabet."

He then goes on to say "Lhuyd, (1707). used χ for ch, λ for ll, and ꝺ for dd. The last has survived in the form ẟ in ordinary handwriting, but manuscript ẟ is printed dd." Unfortunately the symbol ꝺ doesnt show up but in fact in a written version ð is what he means so I guess we may have found our document

This is definately an interesting topic which I'd not actually paid much thought to.

Dysgwr wrote:He then goes on to say "Lhuyd, (1707). used χ for ch, λ for ll, and ꝺ for dd. The last has survived in the form ẟ in ordinary handwriting, but manuscript ẟ is printed dd." Unfortunately the symbol ꝺ doesnt let me see if he actually means ð or not

It seems some of the letters aren't well translated onto the article. There's quite a few ꝺs scattered around. I suspect that ð is meant, though have no way of telling. Except by perhaps finding out who Lhuyd is (Lloyd?).

It seems our Lhuyd is Edward Lhuyd, writing his Archaeologia Britannica: an Account of the Languages, Histories and Customs of Great Britain, from Travels through Wales, Cornwall, Bas-Bretagne, Ireland and Scotland. in 1707. A title that just rolls off the tongue

But in the examples you posted earlier, the page title reads, Testament Newydd (1567). Therefore, it seems your examples are from William Salesbury's New Testament. John Morris Jones has this to say about Mr. Salesbury.

That leads me to believe it was not a widespread phenomenon but more an attempt by individuals at stantardising the language. Its would be interesting to know why these people chose ð when old english had, apparently, died out by the end of the 12th century. It is, also, likely that these people were well educated and would thus have had access to various sources and knowledge enough to make the connections between the various languages with which they were familiar.

treegod wrote:Before that written standards could vary (as has been explained), which might have included <ð>, though perhaps due to the whims of an individual than traditional writing.

Dysgwr wrote:That leads me to believe it was not a widespread phenomenon but more an attempt by individuals at stantardising the language. Its would be interesting to know why these people chose ð when old english had, apparently, died out by the end of the 12th century. It is, also, likely that these people were well educated and would thus have had access to various sources and knowledge enough to make the connections between the various languages with which they were familiar.

I think ð was still used in Middle English, so it's use isn't that unusual.

Interesting idea by Salesbury going the etymological route rather than the phonological route. Interesting in that it's not exactly typical. Welsh would have looked very different (not sure if I'm keen on it really though, lol).